The present invention relates to a fluidized bed reactor for polymerizing or copolymerizing olefins. More particularly, the invention relates to formulation of the bottom part of such a fluidized bed reactor. The invention also relates to a method for polymerizing olefins in a fluidized bed reactor.
Fluidized bed reactors are generally used in continuous-action gas-phase polymerization processes for producing olefin polymers. In a fluidized bed reactor, the polymerization is accomplished in a fluidized bed formed by the polymerization of polymer particles. The bed is maintained in a fluidizing state with the aid of a circulation gas flow directed upwards from the bottom part of the reactor. The circulation gas flow includes gaseous hydrocarbon diluting agents and/or inert gases. In addition, the monomers to be polymerized may be added into the circulating gas line. The circulating gas flow is removed from the gas volume at the top part of the reactor and conducted into thermal exchangers for conveying off the heat produced during the polymerization. Thereafter, the gas flow is returned to the lower part of the reactor with the aid of a compressor.
It is important that the circulating gas in the fluidized bed conducted to the lower section of the reactor be uniformly distributed in order to maintain a uniform fluidization state. Flow division means in the form of a perforated intermediate floor is generally used for distributing the circulating gas. The division means is located adjacent to the lower section of the reactor. Thus, an inlet or mixing chamber for the circulating gas is formed in the bottom part of the reactor and is separated from the fluidized bed by the flow division plate, i.e., separated from the actual polymerization section of the reactor.
It is more difficult to uniformly distribute the circulating gas in the fluidized bed onto the entire cross-section area of the reactor as the reaction size increases. As a result of an uneven distribution, denser and less well fluidizing areas are created, particularly in the vicinity of the reactor walls. This problem is worse when liquid fractions enter along with the circulating gas. This makes it particularly difficult to distribute the liquid phase uniformly within the fluidized bed. Therefore, local heating and agglomeration of polymer particles into larger lumps occur in the fluidized bed, as well as the catching of agglomerates onto the reactor surfaces.
For improving the distribution of the gas flow, prior art devices use gas division plates in which the size, shape and positioning of apertures have been modified. However, it is a significant drawback of such prior art devices that production of these specifically structured gas division plates is expensive and the gas permeability may be inadequate, thus causing unneeded pressure drops in the gas circulation flowing through the reactor.
A second common problem related to the increased fluidized bed reactor size is the agglomerization of polymer particles and their catching on the wall surfaces of the bottom section of the reactor. Some small polymer particles containing an active catalyst tend to pass from the reactor together with the circulating gas. These particles return to the bottom part of the reactor with the circulating gas. If the circulating gas is conducted into the reactor in a conventional manner through a straight tubular connector placed in the bottom of the reactor and, if the flow conditions or the shape of the bottom section of the reactor are not optimal, local flows are produced in the circulating gas inlet chamber adjacent to the area where polymer particles are gathered.
In order to eliminate these drawbacks, some suggestions have been made as to the use of various flow spreading means in conjunction with the circulation gas feed tube. Thus, for instance, in U.S. Pat. No. 4,877,587, dispersion means have been attached on one end of the circulation gas feed tube in the bottom of the reactor to separate the flow from the tube into two parts so that part of the flow turns outward and the rest of the flow goes upwards. With designs such as these, it has not been entirely possible to avoid flows circulating in one place from occurring in the bottom part of the reactor. This results in agglomeration and the catching of polymer particles onto the walls. Other drawbacks include the fact that cleaning the dispersion means is a cumbersome procedure and if one wants, for one reason or another, to employ a different design, dismounting and replacing the dispersion means is difficult and requires a shut-down and an opening of the reactor.
A typical bottom shape of a fluidized bed reactor forms a more or less spherical surface, for instance the device described in U.S. Pat. No. 4,877,587. An advantage of this typical bottom shape in comparison with a planar bottom, is that there will not be any areas having sharp angles produced in the vicinity of the lower part of the wall where, consequently, the gas flow is poor and, therefore, any such sharp angles would cause agglomeration of polymer particles. An arrangement wherein the circulating gas is conducted into a reactor through an inlet aperture or tube located in the center point of the bottom of the reactor, would be unsuccessful in reactors of production scale in which the cross-sectional area of the bottom of the reactor might be well over several meters.